Transition-metal oxides-coated hydrogen-terminated diamond surface and uses thereof

ABSTRACT

The present invention provides a conducting material comprising a carbon-based material selected from a diamond or an insulating diamond-like carbon, having a hydrogen-terminated surface and a layer of tungsten trioxide, rhenium trioxide, or chromium oxide coating said hydrogen-terminated surface. Such conducting materials are useful in the fabrication of electronic components, electrodes, sensors, diodes, field effect transistors, and field emission electron sources.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application No. PCT/IL2018/050971 filed Sep. 2, 2018,designating the U.S. and published as WO 2019/043715 on Mar. 7, 2019which claims the benefit of U.S. Provisional Patent Application No.62/553,871 filed Sep. 3, 2017. Any and all applications for which aforeign or domestic priority claim is identified above and/or in theApplication Data Sheet as filed with the present application are herebyincorporated by reference under 37 CFR 1.57.

TECHNICAL FIELD

The present invention relates to a conductive diamond surface, moreparticularly to tungsten trioxide (WO₃), rhenium trioxide (ReO₃), orchromium oxide (CrO₃)-coated hydrogen-terminated diamond surface showingthermal stability with superior sheet conductivity and hole carrierconcentration, and to uses thereof.

BACKGROUND ART

The next-generation of outperforming electronic devices is predicted tobe led by diamond implementation as extreme semiconductors, as they havethe most favorable properties among other semiconductors for this task(i.e., highest breakdown field and thermal conductivity, dielectricconstant, and carrier transport properties). Up to date, due to the lackof suitable bulk dopants (acceptors and donors) with low activationenergies (Kalish, 2007), diamond free carriers are found only at thesurface and are generated by an unique sub-surface two-dimensional holegas (2DHG) (Strobel et al., 2004; Chakrapani et al., 2007). This 2DHG isformed by an upward band bending (BB), following electron chargetransfer from a hydrogen-terminated diamond (diamond:H) valence bandmaximum (VBM) to a lower unoccupied state of a surface electron acceptor(i.e., >4.2 eV from vacuum level). This induced p-type surfaceconductivity has first been seen in air-exposed hydrogen-terminateddiamond (Landstrass and Ravi, 1989) and has been modeled by anelectrochemical charge-exchange at the interface called transfer doping(TD) (Maier et al., 2000).

This elementary air-exposed surface TD has opened-up initial realizationof diamond-based electronic devices (Umezawa et al., 1999; Taniuchi etal., 2001; Kasu et al., 2005; Kasu et al., 2007), nanoscale planar dopedelectronic devices (Sussmann, 2009; Geisler and Hugel, 2010),electrochemical electrodes (Poh et al., 2004; Christiaens et al., 2006),and biological electrodes sensors (Yang et al., 2002; Lud et al., 2006;Dankerl et al., 2011), to name just a few. In light of this widespectrum of promising applications, a lot of efforts are invested forrecognizing new surface electron acceptors with ability to surpassexisting fragile surface TD of diamond:H. Among all the exposed surfaceacceptor candidates studied so far, none have yet offered a satisfactoryand robust solution for assuring sustainability and efficiency of thediamond surface conductivity as they are required for practicalelectronic device processing and reliability.

First attempts to improve aqueous (i.e., H₂O) air-exposed diamond:Hconductivity performances (Sauerer et al., 2001) involved the use ofalternative adsorbate molecules with high electron affinity (Strobel etal., 2006; Qi et al., 2007). Nevertheless, all those proposed surfaceelectron acceptor molecules were still suffering from instability totemperature fluctuations, resulting in loss of conductivity upon heating(Laikhtman et al., 2004). Thus, the critical need to find more stableand efficient surface acceptor substitutes.

A second approach proposes to encapsulate the above fragile surfaceacceptors with thick oxide layers (i.e., Al₂O₃) in order to protect thefirst active electron acceptor adsorbates on the diamond:H surface(i.e., induced by air) (Kawarada, 2014; Daicho et al., 2014). It hasbeen further developed by using thick capsuling oxide layers (100 nm)with high work functions (e.g., Nb₂O₅, WO₃, V₂O₅, MoO₃ vs. Al₂O₃) inorder to exploit them as additional active layers over the air-dopeddiamond:H surface (Verona et al., 2016). A recent AlN capping layer workhas achieved a hole carrier concentration value of 1×10¹⁴ cm⁻² usingH₂+NH₃ (Imura et al., 2017) activation of diamond:H, and comparablevalues from 8.2×10¹³ cm⁻² to 1.7×10¹⁴ cm⁻² have been reported for a NO₂activation (Wade et al., 2017; Sato and Kasu, 2013). However, despitethe ameliorations, all the above approaches have considerable limitationsuch as in the design, process, and architecture of diamond:H-basedelectronic devices because the resulting 2DHG conductive channel must beprotected with an additional impeding capsulation layer.

Recent advances were reported in the surface TD of diamond:H, usinguniquely transition-metal oxides with high work functions as directexposed surface electron acceptors (without capping), such as MoO₃(φ=5.6-6.8 eV) and V₂O₅ (φ=7 eV) (Russell et al., 2013; Tordjman et al.,2014; Crawford et al., 2016). In these cases, diamond:H/MoO₃ (Tordjmanet al., 2014) demonstrated a hole carrier concentration up to 1×10¹⁴cm⁻², and diamond:H/V₂O₅ (Tordjman et al., 2014) displayed valuescomparable to those of air-exposed diamond:H (Sauerer et al., 2001)(e.g., 1.8×10¹³ cm⁻²) but with a thermal stability of up to 300° C. Inboth cases, oxygen deficiency upon exposure to air is critical anddegrades their original efficient work function (Irfan et al., 2012;Greiner et al., 2012). Consequently, the first active monolayer has tobe buried with additional protecting layers (e.g. 50 Å for MoO₃ and 100Å for V₂O₅) in order to overcome this lacunae. Hence, the crucial needis to find alternative candidates with higher efficiency and thermalstability at the very first monolayer of coverage. Such a desirable casewill ensure the close proximity of electrical top-contacts to thesub-surface 2DHG conductive channel (rich in carrier concentration) andwill cancel the need for additional encumbering protective layers.

SUMMARY OF INVENTION

In one aspect, the present invention provides a conducting materialcomprising a carbon-based material selected from a diamond or insulatingdiamond-like carbon, having a hydrogen-terminated surface and a layer oftungsten trioxide (WO₃), rhenium trioxide (ReO₃), or chromium oxide(CrO₃) coating said hydrogen-terminated surface.

In another aspect, the present invention relates to an electroniccomponent comprising a conducting material as defined above, i.e., aconducting material comprising a carbon-based material having ahydrogen-terminated surface and a layer of WO₃, ReO₃, or CrO₃ coatingsaid hydrogen-terminated surface.

In still another aspect, the present invention relates to an electrodecomprising a conducting material as defined above, i.e., a conductingmaterial comprising a carbon-based material having a hydrogen-terminatedsurface and a layer of WO₃, ReO₃, or CrO₃ coating saidhydrogen-terminated surface.

In yet another aspect, the present invention relates to a sensorcomprising a conducting material as defined above, i.e., a conductingmaterial comprising a carbon-based material having a hydrogen-terminatedsurface and a layer of WO₃, ReO₃, or CrO₃ coating saidhydrogen-terminated surface. Such sensors can be used for detectingchemical and biological materials. In still other aspects, the presentinvention thus relates to the use of such a sensor for the detection ofa chemical or biological material; and to a method for the detection ofa chemical or biological material utilizing said sensor.

In another aspect, the present invention relates to a diode comprising aconducting material as defined above, i.e., a conducting materialcomprising a carbon-based material having a hydrogen-terminated surfaceand a layer of WO₃, ReO₃, or CrO₃ coating said hydrogen-terminatedsurface.

In still another aspect, the present invention relates to a FETcomprising a conducting material as defined above, i.e., a conductingmaterial comprising a carbon-based material having a hydrogen-terminatedsurface and a layer of WO₃, ReO₃, or CrO₃ coating saidhydrogen-terminated surface.

In yet another aspect, the present invention relates to a field emissionelectron source, e.g., a field emission cold cathode, comprising aconducting material as defined above, i.e., a conducting materialcomprising a carbon-based material having a hydrogen-terminated surfaceand a layer of WO₃, ReO₃, or CrO₃ coating said hydrogen-terminatedsurface.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows sheet hole carrier concentration as measured (underidentical conditions) by Hall effect of transfer doped hydrogenterminated diamond with ReO₃ and WO₃, as a function of incrementalsurface acceptor thickness coverage, and H₂O-air-exposed (sphere) resultpresented as reference. (Inset) Transfer doping efficiency vs. surfaceacceptor thickness at close diamond interface proximity for eachrespective transition-metal oxide.

FIGS. 2A-2C show sheet hole carrier concentration (2A); mobility (2B);and conductance (2C) of transfer doped hydrogen terminated diamondsamples with ReO₃ and WO₃ for different thicknesses as function oftemperature (K).

FIGS. 3A-3D show C1s diamond bulk binding energy position as a functionof incremental ReO₃ and WO₃ films thicknesses (6 Å to 45 Å) deposited onhydrogenated diamond (3A); variation of E_(F)−E_(VBM) values as afunction of ReO₃ and WO₃ thickness coverage derived from XPS measurementof C1s diamond bulk binding energy position (3B); and the Re4f and W4fXPS spectra plots of the respective deposited ReO₃ and WO₃ filmthicknesses showing the Re⁶⁺ and W⁶⁺ oxidation state structuresrepresented in dotted lines vs. a continuous line for raw data (3C and3D, respectively).

FIG. 4 schematically illustrates the energy-level diagram before chargeexchange (left) and after equilibrium (right) of the surface transferdoping of hydrogenated diamond with ReO₃ and WO₃. A modified 2DHGdepth-lying is illustrated following the corresponding band bendingvalues and different transfer-doping efficiency of ReO₃ and WO₃.

FIGS. 5A-5B show C1s XPS spectra for incremental ReO₃ (5A) and WO₃ (5B)films thicknesses (6 Å to 45 Å) deposited on hydrogenated diamond.Marked are: C—C bonds from bulk diamond component, the surface componentC—H, a surface component contaminant hydrocarbon C—H, carboxyl weakfeature C—O, sum of fitted peaks (black dots) and (gray line)experimental data (almost overlapping).

FIG. 6 shows a schematic cross-section of starting heterostructure(left) and p-type diamond:H/WO₃ MOSFET (right).

FIGS. 7A-7B show output (7A) and transfer (7B) characteristics forL_(g)=4 diamond:H/WO₃ FETs with WO₃ thickness ranging from 2.4 nm to 4.8nm.

FIGS. 8A-8D show electrical characteristics of 4.8 nm-thick WO₃diamond:H/WO₃ FETs with gate width of 20 μm and different gate lengthsranging from 0.7 μm to 5 μm: (8A) subthreshold and gate currentcharacteristics; (8B) g_(m) characteristics; (8C) maximum g_(m)(g_(mmax)) and maximum drain current (I_(Dsat)), all at V_(DS)=−6 V;(8D) ON resistance.

FIGS. 9A-9B show output (9A) and transfer (9B) characteristics forL_(g)=5 μm WO_(3=4.8) nm diamond:H/WO₃ FETs measured at 77 K and roomtemperature (RT).

FIG. 10 shows split C-V measurements at 1 MHz of a typical diamond:HAVO3FET (L_(g)=5 μm, W_(g)=20 μm, WO₃=4.8 nm) at 77 K. Inset: sheet holemobility vs. sheet hole concentration.

DETAILED DESCRIPTION

It has now been found, in accordance with the present invention, that anadvanced charge-transfer yield is demonstrated by employing singlemonolayers of the transition-metal oxides WO₃ and ReO₃ deposited onhydrogenated diamond surface, resulting in improved p-type sheetconductivity and thermal stability. Surface conductivities, asdetermined by Hall effect measurements as function of temperature forWO₃ yield a record sheet hole carrier concentration value up to2.52×10¹⁴ cm⁻² at room temperature for only few monolayers of coverage.Transfer doping with ReO₃ exhibits a consistent narrow sheet carrierconcentration range value of around 3×10¹³ cm⁻², exhibiting thermalstability up to 450° C. These enhanced conductivity and temperaturerobustness exceed those reported for previous exposed surface electronacceptor materials used so far on diamond surface. X-ray photoelectronspectroscopy (XPS) measurements of the C1s core level shift as functionof WO₃ and ReO₃ layer thicknesses are used to determine the respectiveincrease in surface band bending of the accumulation layers, leading toa different sub-surface two-dimensional hole gas formation efficiency inboth cases. This substantial difference in charge-exchange efficiency isunexpected since both surface acceptors have very close work functionsvalues. Transfer doping with WO₃ reveals the highest yet reportedtransfer doping efficiency per minimal surface acceptor coverage. Thisimproved surface conductivity performance and thermal stability willpromote the realization of 2D diamond-based electronic devices facingprocess fabrication challenges.

In one aspect, the present invention thus provides a conducting materialcomprising a carbon-based material selected from a diamond or insulatingdiamond-like carbon, having a hydrogen-terminated surface and a layer oftungsten trioxide (WO₃), rhenium trioxide (ReO₃), or chromium oxide(CrO₃) coating said hydrogen-terminated surface.

The term “diamond” refers to a carbon-based material which is almostalways found in the crystalline form with a purely cubic orientation ofspa bonded carbon atoms, i.e., to a carbon-based material in which eachcarbon atom is covalently bonded to four other carbon atoms in atetrahedron. Particular diamonds include polycrystalline diamonds,nanocrystalline diamonds, ultra-nanocrystalline diamonds, andhomoepitaxial single diamonds, each optionally doped by boron, nitrogen,hydrogen, phosphorus, or a combination thereof. In certain embodiments,the diamond is a homoepitaxial single crystal diamond, particularly anundoped homoepitaxial single crystal diamond type IIa.

The term “diamond-like carbon” as used herein refers to an amorphouscarbon phase having sp³ hybridized bonds as well as a certain amount ofsp² hybridized bonds, more particularly, a material that has high sp³hybridized bonds content and displays some of the physical properties ofdiamond. Diamond-like carbons exist in seven different forms ofamorphous carbon materials, all containing significant amounts of sp³hybridized carbon atoms. The different forms of diamond-like carbons maybe produced by mixing the two crystalline polytypes of diamond, i.e.,the one having a cubic lattice and the other rare one having a hexagonallattice, in various ways at the nanoscale level of structure, and thesematerials may therefore be at the same time amorphous, flexible, and yetpurely sp³ bonded “diamond”. The hardest, strongest, and slickestmixture is that known as tetrahedral amorphous carbon, or ta-C,considered to be the “pure” form of diamond-like carbon, as it consistsof sp³ bonded carbon atoms only. Diamond-like carbons close enough tota-C on plots of bonding ratios and hydrogen content can be insulatorswith high values of resistivity. The term “insulating diamond-likecarbon” as used herein thus refers to a diamond-like carbon having about50% or more, e.g., about 50%, 55%, 60%, 65%, 70% 75%, 80%, 85%, 90%, 95%or more, sp³ hybridized bonds. It should be noted that where particularvalues are described in the description and claims, unless otherwisestated, the term “about” means that an acceptable error range, e.g., upto 5% or 10%, for the particular value should be assumed.

In a particular such aspect, the present invention provides a conductingmaterial comprising a diamond having a hydrogen-terminated surface and alayer of WO₃, ReO₃, or CrO₃ coating said surface, wherein said diamondmay be a polycrystalline diamond, nanocrystalline diamond,ultra-nanocrystalline diamond, or homoepitaxial single diamond, undopedor doped by boron, nitrogen, hydrogen, phosphorus, or a combinationthereof, e.g., an undoped homoepitaxial single crystal diamond type IIa.

In certain embodiments, the WO₃, ReO₃ or CrO₃ layer coating thehydrogen-terminated surface of the carbon-based material is asingle-layer or a multi-layer of said oxide having a thickness in arange of 5 Å to 1000 Å, 5 Å to 900 Å, 5 Å to 800 Å, 5 Å to 700 Å, 5 Å to600 Å, 5 Å to 500 Å, 5 Å to 450 Å, 5 Å to 400 Å, 5 Å to 350 Å, 5 Å to300 Å, 5 Å to 250 Å, 5 Å to 200 Å, 5 Å to 150 Å, or 5 Å to 100 Å.

In some embodiments, the conducting material of the invention comprisesa carbon-based material selected from a diamond or insulatingdiamond-like carbon, having a hydrogen-terminated surface and a layer ofWO₃ coating said hydrogen-terminated surface. In particular suchembodiments, said WO₃ layer is a single- or multi-layer of WO₃ having athickness in a range of 5 Å to 1000 Å, 5 Å to 900 Å, 5 Å to 800 Å, 5 Åto 700 Å, 5 Å to 600 Å, 5 Å to 500 Å, 5 Å to 450 Å, 5 Å to 400 Å, 5 Å to350 Å, 5 Å to 300 Å, 5 Å to 250 Å, 5 Å to 200 Å, 5 Å to 150 Å, or 5 Å to100 Å. Preferred such embodiments are those wherein said WO₃ layer has athickness in the range of 5 Å to 150 Å, or 5 Å to 100 Å.

In other embodiments, the invention provides a conducting materialcomprising a carbon-based material selected from a diamond or insulatingdiamond-like carbon, having a hydrogen-terminated surface and a layer ofReO₃ coating said hydrogen-terminated surface. In particular suchembodiments, said ReO₃ layer is a single- or multi-layer of ReO₃ havinga thickness in a range of 5 Å to 1000 Å, 5 Å to 900 Å, 5 Å to 800 Å, 5 Åto 700 Å, 5 Å to 600 Å, 5 Å to 500 Å, 5 Å to 450 Å, 5 Å to 400 Å, 5 Å to350 Å, 5 Å to 300 Å, 5 Å to 250 Å, 5 Å to 200 Å, 5 Å to 150 Å, or 5 Å to100 Å. Preferred embodiments are those wherein said ReO₃ layer has athickness in the range of 5 Å to 150 Å, or 5 Å to 100 Å.

In further embodiments, the invention provides a conducting materialcomprising a carbon-based material selected from a diamond or insulatingdiamond-like carbon, having a hydrogen-terminated surface and a layer ofCrO₃ coating said hydrogen-terminated surface. In particular suchembodiments, said CrO₃ layer is a single- or multi-layer of CrO₃ havinga thickness in a range of 5 Å to 1000 Å, 5 Å to 900 Å, 5 Å to 800 Å, 5 Åto 700 Å, 5 Å to 600 Å, 5 Å to 500 Å, 5 Å to 450 Å, 5 Å to 400 Å, 5 Å to350 Å, 5 Å to 300 Å, 5 Å to 250 Å, 5 Å to 200 Å, 5 Å to 150 Å, or 5 Å to100 Å. Preferred embodiments are those wherein said CrO₃ layer has athickness in the range of 5 Å to 150 Å, or 5 Å to 100 Å.

In certain embodiments, the carbon based material composing theconducting material of the present invention is a diamond, in any one ofthe forms listed above, and the thickness of the WO₃ layer coating thehydrogen-terminated surface of the diamond is in a range of 5 Å to 1000Å, 5 Å to 900 Å, 5 Å to 800 Å, 5 Å to 700 Å, 5 Å to 600 Å, 5 Å to 500 Å,5 Å to 450 Å, 5 Å to 400 Å, 5 Å to 350 Å, 5 Å to 300 Å, 5 Å to 250 Å, 5Å to 200 Å, 5 Å to 150 Å, or 5 Å to 100 Å, but preferably in the rangeof 5 Å to 150 Å, or 5 Å to 100 Å. As shown herein, such conductingmaterials have an electrical stability of up to 300° C. In particularsuch embodiments, the conducting material of the invention has a sheetconductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to10⁻²Ω⁻¹; and a hole carrier concentration higher than 1×10¹¹ cm⁻², withthickness and temperature dependency, or higher than 10¹³ cm⁻² at roomtemperature, e.g., in a range of 10¹¹ cm⁻² to 3×10¹⁴ cm⁻².

In certain embodiments, the carbon based material composing theconducting material of the present invention is a diamond, in any one ofthe forms listed above, and the thickness of the ReO₃ layer coating thehydrogen-terminated surface of the diamond is in a range of 5 Å to 1000Å, 5 Å to 900 Å, 5 Å to 800 Å, 5 Å to 700 Å, 5 Å to 600 Å, 5 Å to 500 Å,5 Å to 450 Å, 5 Å to 400 Å, 5 Å to 350 Å, 5 Å to 300 Å, 5 Å to 250 Å, 5Å to 200 Å, 5 Å to 150 Å, or 5 Å to 100 Å, but preferably in the rangeof 5 Å to 150 Å, or 5 Å to 100 Å. As shown herein, such conductingmaterials have an electrical stability of up to 450° C. In particularsuch embodiments, the conducting material of the invention has a sheetconductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to10⁻²Ω⁻¹; and a hole carrier concentration higher than 10¹² cm⁻², withthickness and temperature dependency, or higher than 10¹³ cm⁻² at roomtemperature, e.g., in a range of 10¹² cm⁻² to 10¹⁴ cm⁻².

In certain embodiments, the carbon based material composing theconducting material of the present invention is a diamond, in any one ofthe forms listed above, and the thickness of the CrO₃ layer coating thehydrogen-terminated surface of the diamond is in a range of 5 Å to 1000Å, 5 Å to 900 Å, 5 Å to 800 Å, 5 Å to 700 Å, 5 Å to 600 Å, 5 Å to 500 Å,5 Å to 450 Å, 5 Å to 400 Å, 5 Å to 350 Å, 5 Å to 300 Å, 5 Å to 250 Å, 5Å to 200 Å, 5 Å to 150 Å, or 5 Å to imA, but preferably in the range of5 Å to 150 Å, or 5 Å to 100 Å. Such conducting materials have anelectrical stability of up to 450° C. In particular such embodiments,the conducting material of the invention has a sheet conductance higherthan 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹; and a holecarrier concentration higher than 10¹² cm⁻², with thickness andtemperature dependency, or higher than 10¹³ cm⁻² at room temperature,e.g., in a range of 10¹² cm⁻² to 10¹⁴ cm⁻².

In another aspect, the present invention relates to an electroniccomponent comprising a conducting material as defined in any one of theembodiments above, i.e., a conducting material comprising a carbon-basedmaterial having a hydrogen-terminated surface and a layer of WO₃, ReO₃,or CrO₃ coating said hydrogen-terminated surface. In certainembodiments, the electronic component of the invention comprises aconducting material wherein the carbon-based material is a diamond inany one of the forms listed above, and the thickness of the oxide layercoating the hydrogen-terminated surface of the diamond is in a range of5 Å to 1000 Å, but preferably 5 Å to 150 Å, or 5 Å to 100 Å. Particularsuch electronic components are those wherein said diamond is coated with(i) a WO₃ layer having a thickness in a range of 5 Å to 1000 Å, whereinsaid conducting material has a sheet conductance higher than 1×10⁻⁵Ω⁻¹,e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentrationhigher than 1×10¹¹ cm⁻², e.g., in a range of 10¹¹ cm⁻² to 3×10¹⁴ cm⁻²;(ii) a ReO₃ layer having a thickness in a range of 5 Å to 1000 Å,wherein said conducting material has a sheet conductance higher than1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrierconcentration higher than 10¹² cm⁻², e.g., in a range of 10¹² cm⁻² to10¹⁴ cm⁻²; or (iii) a CrO₃ layer having a thickness in a range of 5 Å to1000 Å, wherein said conducting material has a sheet conductance higherthan 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a holecarrier concentration higher than 10¹² cm⁻², e.g., in a range of 10¹²cm⁻² to 10¹⁴ cm⁻².

In still another aspect, the present invention relates to an electrodecomprising a conducting material as defined in any one of theembodiments above, i.e., a conducting material comprising a carbon-basedmaterial having a hydrogen-terminated surface and a layer of WO₃, ReO₃,or CrO₃ coating said hydrogen-terminated surface. In certainembodiments, the electrode of the invention comprises a conductingmaterial wherein the carbon-based material is a diamond in any one ofthe forms listed above, and the thickness of the oxide layer coating thehydrogen-terminated surface of the diamond is in a range of 5 Å to 1000Å, but preferably 5 Å to 150 Å, or 5 Å to 100 Å. Particular suchelectrodes are those wherein said diamond is coated with (i) a WO₃ layerhaving a thickness in a range of 5 Å to 1000 Å, wherein said conductingmaterial has a sheet conductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a rangeof 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentration higher than1×10¹¹ cm⁻²e.g., in a range of 10¹¹ cm⁻² to 3×10¹⁴ cm⁻²; (ii)) a ReO₃layer having a thickness in a range of 5 Å to 1000 Å, wherein saidconducting material has a sheet conductance higher than 1×10⁻⁵Ω⁻¹, e.g.,in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentrationhigher than 10¹² cm⁻², e.g., in a range of 10¹² cm⁻² to 10¹⁴ cm⁻²; or(iii) a CrO₃ layer having a thickness in a range of 5 Å to 1000 Å,wherein said conducting material has a sheet conductance higher than1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrierconcentration higher than 10¹² cm⁻², e.g., in a range of 10¹² cm⁻² to10¹⁴ cm⁻². According to the invention, the electrode may comprise saidconducting material as a conductive layer only or, alternatively, mayentirely consist of said conducting material.

In yet another aspect, the present invention relates to a sensorcomprising a conducting material as defined above, i.e., a conductingmaterial comprising a carbon-based material having a hydrogen-terminatedsurface and a layer of WO₃, ReO₃, or CrO₃ coating saidhydrogen-terminated surface. In certain embodiments, the sensor of theinvention comprises a conducting material wherein the carbon-basedmaterial is a diamond in any one of the forms listed above, and thethickness of the oxide layer coating the hydrogen-terminated surface ofthe diamond is in a range of 5 Å to 1000 Å, but preferably 5 Å to 150 Å,or 5 Å to 100 Å. Particular such sensors are those wherein said diamondis coated with (i) a WO₃ layer having a thickness in a range of 5 Å to1000 Å, wherein said conducting material has a sheet conductance higherthan 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a holecarrier concentration higher than 1×10¹¹ cm⁻², e.g., in a range of 10¹¹cm⁻² to 3×10¹⁴ cm⁻²; (ii) a ReO₃ layer having a thickness in a range of5 Å to 1000 Å, wherein said conducting material has a sheet conductancehigher than 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and ahole carrier concentration higher than 10¹² cm⁻², e.g., in a range of10¹² cm⁻² to 10¹⁴ cm⁻²; or (iii) a CrO₃ layer having a thickness in arange of 5 Å to 1000 Å, wherein said conducting material has a sheetconductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to10⁻²Ω⁻¹, and a hole carrier concentration higher than 10¹² cm⁻², e.g.,in a range of 10¹² cm⁻² to 10¹⁴ cm⁻².

The sensor of the invention may be used, inter alia, for the detectionof chemical or biological materials. In still other aspects, the presentinvention thus relates to the use of such a sensor for the detection ofa chemical or biological material; and to a method for the detection ofa chemical or biological material utilizing such a sensor.

A diode is a two-terminal electronic component with an asymmetrictransfer characteristic, with low (ideally zero) resistance to currentflow in one direction, and high (ideally infinite) resistance in theother. A semiconductor diode, the most common type today, is acrystalline piece of semiconductor material with a p-n junctionconnected to two electrical terminals. A p-n junction is a boundary orinterface between two types of semiconductor material, p-type (a dopedsemiconductor containing excess holes) and n-type (doped semiconductorcontaining excess free electrons), inside a single crystal ofsemiconductor, created by doping, e.g., by ion implantation, diffusionof dopants, or by epitaxy, i.e., growing a layer of crystal doped withone type of dopant on top of a layer of crystal doped with another typeof dopant. P-n junctions are elementary “building blocks” of mostsemiconductor electronic devices such as diodes, transistors,light-emitting diodes (LEDs) and integrated circuits, and are the activesites where the electronic action of the device takes place. Certainelectronic devices such as particular types of transistors, e.g.,bipolar junction transistors, consist of two pn junctions in series, inthe form of npn or pnp jubction or heteroj uncti on.

In another aspect, the present invention relates to a diode comprising aconducting material as defined in any one of the embodiments above,i.e., a conducting material comprising a carbon-based material having ahydrogen-terminated surface and a layer of WO₃, ReO₃, or CrO₃ coatingsaid hydrogen-terminated surface. In certain embodiments, the diode ofthe invention comprises a conducting material wherein the carbon-basedmaterial is a diamond in any one of the forms listed above, and thethickness of the oxide layer coating the hydrogen-terminated surface ofthe diamond is in a range of 5 Å to 1000 Å, but preferably 5 Å to 150 Å,or 5 Å to 100 Å. Particular such diodes are those wherein said diamondis coated with (i) a WO₃ layer having a thickness in a range of 5 Å to1000 Å, wherein said conducting material has a sheet conductance higherthan 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a holecarrier concentration higher than 1×10¹¹ cm⁻², e.g., in a range of 10¹¹cm⁻² to 3×10¹⁴ cm⁻²; (ii) a ReO₃ layer having a thickness in a range of5 Å to 1000 Å, wherein said conducting material has a sheet conductancehigher than 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and ahole carrier concentration higher than 10¹² cm⁻², e.g., in a range of10¹² cm⁻² to 10¹⁴ cm⁻²; or (iii) a CrO₃ layer having a thickness in arange of 5 Å to 1000 Å, wherein said conducting material has a sheetconductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to10⁻²Ω⁻¹, and a hole carrier concentration higher than 10¹² cm⁻², e.g.,in a range of 10¹² cm⁻² to 10¹⁴ cm⁻².

In certain embodiments, the diode of the invention is a p-n junctiondiode, a p-n-p heterojunction diode, or n-p-n heterojunction diode,wherein one or more of the p-type layers in said diode comprises saidconducting material and/or said conducting material bridges said p-njunction, p-n-p heterojunction or n-p-n heterojunction.

In other embodiments, the diode of the invention is configured asSchottky diode, also known as hot carrier diode, in which ametal-semiconductor rather than a semiconductor-semiconductor junctionis formed, creating a Schottky barrier, i.e., as a semiconductor diodewith a low forward voltage drop and a very fast switching action.

Field effect transistor (FET) is a unipolar transistor using an electricfield to control the shape and hence the conductivity of a channel ofone type of charge carrier in a semiconductor material. The deviceconsists of an active channel through which charge carriers, electronsor holes, flow from the source, through which the carriers enter thechannel, to the drain, through which the carriers leave the channel,wherein the conductivity of the channel is a function of the potentialapplied across the gate terminal, i.e., the terminal that modulates thechannel conductivity, and source terminal. A FET can be constructed froma number of semiconductors, wherein the channel is doped to produceeither an n-type or a p-type semiconductor, and the drain and source areeach doped of similar or opposite type to the channel, depending on themode of the FET.

In still another aspect, the present invention relates to a FETcomprising a conducting material as defined in any one of theembodiments above, i.e., a conducting material comprising a carbon-basedmaterial having a hydrogen-terminated surface and a layer of WO₃, ReO₃,or CrO₃ coating said hydrogen-terminated surface. In certainembodiments, the FET of the invention comprises a conducting materialwherein the carbon-based material is a diamond in any one of the formslisted above, and the thickness of the oxide layer coating thehydrogen-terminated surface of the diamond is in a range of 5 Å to 1000Å, but preferably 5 Å to 150 Å, or 5 Å to 100 Å. Particular such FETsare those wherein said diamond is coated with (i) a WO₃ layer having athickness in a range of 5 Å to 1000 Å, wherein said conducting materialhas a sheet conductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a range of10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentration higher than 1×10¹¹cm⁻², e.g., in a range of 10¹¹ cm⁻² to 3×10¹⁴ cm⁻²; (ii) a ReO₃ layerhaving a thickness in a range of 5 Å to 1000 Å, wherein said conductingmaterial has a sheet conductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a rangeof 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentration higher than 10¹²cm⁻², e.g., in a range of 10¹² cm⁻² to 10¹⁴ cm⁻²; or (iii) a CrO₃ layerhaving a thickness in a range of 5 Å to 1000 Å, wherein said conductingmaterial has a sheet conductance higher than 1×10⁻⁵Ω⁻¹, e.g., in a rangeof 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentration higher than 10¹²cm⁻², e.g., in a range of 10¹² cm⁻² to 10¹⁴ cm⁻². The FET of theinvention may comprise said conducting material as a conducting layerand/or as a p-type layer. In one particular embodiment, the FET of theinvention is constructed as a high-frequency high power FET.

Field emission, also known as field electron emission and electron fieldemission, is emission of electrons induced by an electrostatic field,e.g., from a solid surface such as pure metals into vacuum, air, afluid, or any non-conducting or weakly conducting dielectric. Fieldemission is explained by quantum tunneling of electrons, wherein thewave-mechanical tunneling of electrons through a rounded triangularbarrier created at the surface of an electron conductor by applying avery high electric field is known as Fowler-Nordheim tunneling.Individual electrons can escape by Fowler-Nordheim tunneling from manymaterials in different circumstances. Cold field electron emission is astatistical emission regime, in which the electrons in the emitter areinitially in internal thermodynamic equilibrium, and most emittedelectrons escape by Fowler-Nordheim tunneling from electron states closeto the emitter Fermi level, i.e., electrochemical potential.

Cold cathodes are cathodes, i.e., electrodes emitting electrons, whichin contrast to hot cathodes, are electrically heated to their operatingtemperature by methods other than electric current passing through afilament.

In yet another aspect, the present invention relates to a field emissionelectron source, e.g., a field emission cold cathode, comprising aconducting material as defined in any one of the embodiments above,i.e., a conducting material comprising a carbon-based material having ahydrogen-terminated surface and a layer of WO₃, ReO₃, or CrO₃ coatingsaid hydrogen-terminated surface. In certain embodiments, the fieldemission electron source of the invention comprises a conductingmaterial wherein the carbon-based material is a diamond in any one ofthe forms listed above, and the thickness of the oxide layer coating thehydrogen-terminated surface of the diamond is in a range of 5 Å to 1000Å, but preferably 5 Å to 150 Å, or 5 Å to 100 Å. Particular such fieldemission electron sources are those wherein said diamond is coated with(i) a WO₃ layer having a thickness in a range of 5 Å to 1000 Å, whereinsaid conducting material has a sheet conductance higher than 1×10⁻⁵Ω⁻¹,e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentrationhigher than 1×10¹¹ cm⁻², e.g., in a range of 10¹¹ cm⁻² to 3×10¹⁴ cm⁻²;(ii) a ReO₃ layer having a thickness in a range of 5 Å to 1000 Å,wherein said conducting material has a sheet conductance higher than1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrierconcentration higher than 10¹² cm⁻², e.g., in a range of 10¹² cm⁻² to10¹⁴ cm⁻²; or (iii) a CrO₃ layer having a thickness in a range of 5 Å to1000 Å, wherein said conducting material has a sheet conductance higherthan 1×10⁻⁵Ω⁻¹, e.g., in a range of 10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a holecarrier concentration higher than 10¹² cm⁻², e.g., in a range of 10¹²cm⁻² to 10¹⁴ cm⁻².

The invention will now be illustrated by the following non-limitingExamples.

EXAMPLES

Study 1. Boosting Surface Charge-Transfer Doping Efficiency andRobustness of Diamond with WO₃ and ReO₃

Disclosed herein is the use of WO₃ and ReO₃ as direct surface electronacceptor layers for the TD of the diamond:H surface. WO₃ TD is found toyield a p-type conductive surface layer at a close diamond:H interfaceproximity with electrical properties significantly superior to thoseinduced by other materials reported so far (FIG. 1). The areal holedensity at room temperature vs. thickness coverage, as measured by Halleffects, is found to reach a record value of up to 2.52×10¹⁴ cm⁻² at thevery first monolayers of coverage (from 1.2 nm to 4.8 nm) for WO₃. ForReO₃, a value in the range of 2.87-3.63×10¹³ cm⁻² is shown for thedifferent thicknesses. Hall effect measurements as a function oftemperature for diamond:H/ReO₃ layers show an impressive electricalstability of up to at least 450° C., while diamond:H/WO₃ loses its highelectrical performance after 300° C.

In charge-exchange complexes, work function is crucial for determiningthe energetic requirements of transferring electrons from a solid untilelectrodynamic equilibrium. Therefore, transition-metal oxides (TMOs)are attracting considerable interest due to their low-lying conductionband and/or high work functions. Besides having different latticeconstants and crystallinity structures, the electronic properties ofTMOs differ mainly by their sensitivity to oxygen deficiency, whichtranslates to the degradation of work function values and hence to thestability of their charge transfer properties. In the present case, workfunctions of WO₃ (φ=6.7 eV) (Gerling et al., 2015) and ReO₃ (φ=6.7-6.8eV) (Yoo et al., 2014; Suchitra et al., 2014) are very close to eachother and to the aforementioned TMOs; thus, one would have expected tosee similar results. However, in practice, diamond:H surfaceconductivity with the present oxides shows substantially differentelectrical characteristics and performances, suggesting additionalfactors (i.e., beside work function value) influencing thecharge-exchange efficiency at the host/acceptor interlayer (FIG. 1).

The dependence of changes in the electrical and surface propertieschanges with the thickness of the deposited WO₃ and ReO₃ layers ondiamond:H is investigated by carrying out a combination of Hall effectsurface conductivity with C1s core level and corresponding Re(4f) andW(4f) spectra XPS measurements.

Diamond:H samples were coated with thermally evaporated WO₃ and ReO₃with thicknesses ranging from 5 Å to 100 Å, after in-situ heating at350° C., in order to prudently remove any humid-air-related adsorbatesand exclude their eventual contribution to conductivity (seeExperimental section). The electrical properties of all samples weremeasured by the Hall effect in a Van der Pauw configuration as describedelsewhere (Yom-Tov et al., 2010). FIG. 1 shows a comparison of the arealhole density and conductance values for different transition-metal oxidesurface acceptor layers inducing TD of diamond:H vs. their incrementallayer thicknesses, all prepared and measured under identical conditions.A diamond:H TD induced with H₂O from the air exposed is also presentedin FIG. 1 for direct comparison. The data clearly demonstrate superiorelectrical properties obtained for WO₃ layers evaporated onto diamond:Hat the first monolayers of coverage with areal hole concentration valuesranging from 2.52×10¹⁴ cm⁻² to 1.3×10¹⁴ cm⁻², corresponding tothicknesses from 1.2 nm to 4.8 nm. Diamond:H/ReO₃ keeps constant thevalues of sheet hole density in a narrow range of 3.63×10¹³ cm⁻² to2.4×10¹³ cm⁻².

Since the formed 2DHG depth at the diamond:H sub-surface will govern theavailable amount of carrier concentrations measured at the surface, wedefine a TD efficiency coefficient η as:η=100×(E_(F)−E_(VBM))/Thickness, corresponding to the band bending depthvalue, E_(F)−E_(VBM) (extracted from XPS measurement, FIG. 3b ) dividedby the surface electron acceptor thickness. The inset of FIG. 1 showsthe thus defined efficiency plotted vs. surface acceptor coverage anddepicts how much each surface electron acceptors efficiently induces adeep-lying 2DHG at a given surface coverage thickness.

FIGS. 2a-2c show the hole sheet carrier concentration, carrier mobility,and conductance, measured by the Hall effect as a function oftemperature between −200° C. to 450° C. (plotted in K) fordiamond:H/WO₃, and diamond:H/ReO₃ samples with thicknesses ranging from1.2 nm to 6.4 nm. Diamond:H/WO₃ samples show an increase in the sheethole concentration from 1.7-5.77×10¹³ cm⁻² at −200° C. to 2.18-4.78×10¹⁴cm⁻² for a temperature up to 150° C., followed by a moderate decreasewith a sheet hole concentration from 1.7×10¹⁴ cm⁻² to 5.8×10¹³ cm⁻² for300° C. and a loss of two orders of magnitude in the carrierconcentration when reaching 350° C. In contrast, diamond:H/ReO₃ shows anoutstanding stability in the sheet hole carrier concentration within anarrow value in the range of 2.55-3.12×10¹³ cm⁻² from −200° C. to 450°C. Hole mobility of diamond:H/WO₃ ranges between 20 and 104 cm⁻²V⁻¹s⁻¹for temperatures up to 300° C. and increase to a value of 320 cm⁻²V⁻¹s⁻¹at 350° C. following the decreasing carrier concentration. Taking intoaccount the relatively high thickness layer and keeping in mind thecrystallinity phase formation of WO₃ at such temperature, this sharpmobility increase may be tentatively attributed to a carrier transporttransition from a quasi-2DHG to an emerging 3D channel structure (i.e.,WO₃ layer) where the screening of the ionized impurities tends todiminish. Diamond:H/ReO₃ samples show a moderate monotonic increase inhole mobility and in resistivity changing from 20 to 140 cm⁻²V⁻¹s⁻¹ whenthe temperature is changed from −200° C. to 450° C. A note worth to beadded concerning the remarkable conductance value of 1.5×10⁻³ Ohm⁻¹measured for diamond:H/WO₃ (1.2-2.4 nm) at room temperature and thevalue of 2-3×10⁻³ Ohm⁻¹ for diamond:H/WO₃ (4.8-6.4 nm) at 150° C.Surface characterization of chemical bonding and band bending evolutionof the diamond:H/ReO₃ and diamond:H/WO₃ interfaces were determined byXPS measurements of C1s core level spectra of the incremental layerthicknesses. Re(4f) and W(4f) core level spectra were also measured(FIGS. 3c-3d ) for characterizing the stoichiometry of the various ReO₃and WO₃ layers and analyzing eventual oxygen deficiencies, capable ofcausing work function modifications.

XPS spectra of Re(4f) (FIG. 3c ) show the Re⁶⁺4f_(7/2) and 4f_(5/2)doublet peaks (with a splitting binding energy (BE) of 2.4 eVRe⁶⁺4f_(5/2)/Re⁶⁺4f_(7/2) ratio of 3:4) for different thicknesses, atbinding energies of 46.1 eV and 48.7 eV, in agreement with previousreports (Yoo et al., 2014). This pair of peaks represents the Re⁶⁺oxidation state structure (dotted line) and reveals its predominantpresence for different diamond:H/ReO₃ films. Similarly, FIG. 3d showsthe W⁶+4f_(7/2) and 4f_(5/2) doublet peaks (with a splitting BE of 2.1eV and W⁶+4f_(5/2)/W⁶+4f_(7/2) ratio of 3:4) for different thicknesses,appearing at binding energies of 35.7 eV and 37.8 eV, similar toprevious reports (Bertus et al., 2013). Here again, the pair of peaksrepresenting W⁶⁺ oxidation state (dotted line) is the predominantstructure for the incremented different diamond:H/WO₃ films. The absenceof reduction states in different ReO₃ and WO₃ layers excludes possibleoxygen deficiencies that may be the origin of gap state generation closeto the Fermi level and work function degradation. These results arefound to be identical before and after electrical measurements.

The C1s core level spectra for both ReO₃ and WO₃ at differentthicknesses were measured by XPS in order to probe the state of the nearsurface C atoms and to estimate the position of the surface Fermi level(E_(F)) relative to the valence band maximum (E_(VBM)). The results ofC1s core level peaks with their detailed de-convoluted bondingcomponents for diamond:H covered with ReO₃ and WO₃ at increasingthicknesses and, as a reference, for a type IIb boron doped diamond:Hafter annealing at 400° C. (lowest curve) can be found in theExperimental section.

The C1s diamond bulk binding energy position shift as a function of ReO₃and WO₃ thickness coverage is shown in FIG. 3a . The band bending (BB)value, E_(F)−E_(VBM) (schematically represented in FIG. 3b ), wasdetermined from the energy difference between the diamond bulk peak,extracted from the measured C1s binding energy (FIG. 3a ), and the knownfixed energy separation of the valence band maximum (VBM) to the C1score level of 283.9±0.1 eV (Maier et al., 2001). The dependence ofE_(F)−E_(VBM) with ReO₃ and WO₃ thicknesses is shown in FIG. 3b . TheE_(VBM) value starts from a position below the bulk Fermi level (0.35eV), in accord with other reports (Edmonds et al., 2011), and risesrapidly, reaching a maximum of 0.6 and 1 eV above the Fermi level forReO₃ and WO₃ thicknesses of 25 Å and 35 Å, respectively, whereupon asaturation is noted.

Close to the diamond:H interface (FIG. 1b ), the contribution of eachTMO surface acceptor's first monolayer (up to 1.2 nm) to the 2DHGformation is very different from each other. WO₃ clearly excels inyielding the deeper-lying 2DHG for minimum coverage, followed by ReO₃.This same order of superiority is recognized in the carrierconcentration values vs. same thicknesses (FIG. 1a ) of each respectiveTMO. This similar trend appearing on both plots and extracted from bothtechniques (E_(F)−E_(VBM) from XPS vs. carrier concentration from Halleffect) suggests a further confirmation of the present experimentalresult correlation of the electrical measurement and band bendingvalues. For TMO surface acceptor's thicker thicknesses (>5 nm), carrierconcentration values and TD efficiency tend to converge. FIG. 4 showsthe presentation of the energy-level diagram before charge exchange(FIG. 4a ), and after equilibrium (FIG. 4b ) of the surface transferdoping of hydrogenated diamond with ReO₃ and WO₃. As a result of thetransfer of charge, a doping induced interface dipole arises and isexpected to behave like an ideal capacitor—this is presentedschematically and labeled as the active interface.

Despite the close values of work functions for both surface electronacceptor substances used here (ReO₃ and WO₃), an unexpected substantialdifference in the experimental surface conductivity results fordiamond:H/ReO₃ and diamond:H/WO₃ is recorded (FIG. 2). This electricalbehavior is further pronounced by the fact that the oxides give rise todifferent band bending depths (FIG. 3a ) and consequently different 2DHGeffective dimensions (FIG. 4). This suggests that the surface TDefficiency of diamond:H is not only affected by the surface acceptorwork functions, as it has been accepted so far, but also susceptible tobe influenced by additional factors.

TMO crystallinity is temperature dependent and differs from material tomaterial. This temperature dependence will certainly affect theelectrical properties of different dopant layers. ReO₃ exists in asingle phase of primitive cubic structure of cP4 (Greenwood andEarnshaw, 1997). This single and symmetrical unit cell contributes tothe stable lattice structure capabilities against eventualreconstruction upon heating. This may fairly explain the highersustainability of the diamond:H/ReO₃ conductive surface up to 450° C. ascompared to the case of V₂O₅, which also has a single crystalline phase(however, being orthorhombic with an asymmetric unit cell) and has beenpreviously reported to yield a stable surface conductivity of up to 300°C. (Crawford et al., 2016). Oppositely, WO₃ has three differentcrystalline phases—triclinic, monoclinic, and orthorhombic—whichspontaneously reorganize, following the temperature range values of−50-17° C., 17-330° C., and 330-740° C., respectively (Lassner andSchubert, 1999). This may explain the three different electricalproperties of the diamond:H/WO₃ surface we see as function oftemperature (FIG. 2a ), which exhibits higher conductivity values forthe favorable crystalline phase benefitting higher atomic density at theinterface coverage (e.g., as seen for monoclinic up to 300° C.). It cantherefore be postulated that the highest areal hole concentration of4.78×10¹⁴ cm⁻² reported here for the diamond:H/WO₃ surface may beassociated with the high atomic density structure layer offered by thesurface acceptor WO₃ monoclinic phase, formed at the interface withdiamond:H. An analogous comparison can be done with diamond:H/MoO₃ whichhas been reported to yield an areal hole density of up to 1×10¹⁴ cm⁻²for the same temperature range, knowing that MoO₃ tends to crystallizein the orthorhombic structure of α-phase, which has a relatively loweratomic density structure as compared with the monoclinic one (Di Yao etal., 2012). We therefore propose that the results reported here aboutthe magnitude and the temperature stability of the diamond:H transferdoping with WO₃ and ReO₃, are influenced by the combined work functionsand atomic density structure characteristic of the electron surfaceacceptor's layers at the diamond:H/TMO's interface.

In summary, a way for obtaining a p-type surface conductivity bytransfer doping of hydrogen-terminated diamond with minimal WO₃ and ReO₃coverage is reported. The hall effect measurement vs. temperature andcore level C1s XPS measurements were used to depict a higher transferdoping efficiency per thickness coverage for the case of diamond:H/WO₃.The surface conductivity of diamond:H/WO₃ yields a record value of thehole carrier concentration (2.18-4.78×10¹⁴ cm⁻²) at the very firstmonolayer coverage (from 1.2 nm), whereas diamond:H/ReO₃ surfaceconductivity is found to be remarkably stable up to 450° C. Thesefindings suggest that TMO's work functions are not the only factorgoverning the charge-transfer efficiency but may also be influenced bythe atomic density structure characteristics of the film at theinterface. The present study improves the performances of surfaceelectron acceptors used so far and provides a new advantageous means forrealizing electronic devices based on diamond surface conductivity withhigher performances.

Experimental

Samples Preparation

Type IIa (100) diamond single crystal samples were used. Surfacetreatment of the samples include cleaning in boiling acids and hydrogentermination by exposure to pure hydrogen plasma in a CVD reactor at atemperature of about 650° C. for 30 minutes. The samples were thenintroduced to a vacuum chamber (10⁻⁷ torr) for ReO₃ and WO₃ thermalevaporation of various thicknesses from 5 Å to 150 Å. Prior to eachdeposition, the hydrogenated diamond samples were heated, in situ, to350° C. during 60 minutes through an underlying heater, to removehydrocarbon contaminants and to desorb any water adsorbate inducingsurface conductivity during ambient exposure. ReO₃ and WO₃ wereseparately evaporated in situ from a Knudsen cell onto the samplesurfaces at room temperature with a deposition rate of 0.1 nm/min,nominally determined by a quartz crystal microbalance. The depositedReO₃ and WO₃ thicknesses values were confirmed by ellipsometrymeasurements over Si samples references for every deposition batch.

As a verification, a non-hydrogenated diamond, coated with WO₃ and ReO₃(25 Å) under similar conditions, has shown a resistance higher than 10⁵kΩ/Sqr with currents below the detection limit of 100 pA of Hall effectsystems for all temperatures. This control clearly proves the absence ofparallel conduction contributions from the oxides alone without aprevious charge exchange from diamond.

Surface Characterization

Electrical measurements consisting of carrier type, carrierconcentration, and mobility were measured as function of temperature,from −200° C. to 450° C., using Hall Effect measurements with magneticfield up to 1.5 T using a Van der Pauw (VdP) contact configuration. Foursilver symmetric paint corner-points placed on the top layer of thesamples were used for electrical contacts. Hall system data acquisitionand analysis algorithm took into account the required 2D sheetgeometrical rectification following our standard VdP contacts. Thisrectification has routinely been calibrated with a reference p-Si withknown thickness doped layer before measurements. A further similarreference verification is also done with a Boron doped diamond withknown concentration as mentioned elsewhere (Suchitra et al., 2014).Additionally, the same measurements conditions were applied similarlyfor diamond:H/H₂O (FIG. 5), used as well as reference value. Each dataacquisition point values have been received by several round loopmeasurements and have been extracted as final value within an error rateof less than 10% (error bars in FIG. 5). It should be noted, that thescattering mechanisms involved in the presence of a magnetic field aredifferent, therefore the measured Hall mobility is expected to differsomewhat from the drift mobility.

XPS measurements were used to characterize the chemical bonding and todetermine the band structure of the films. These measurements wereconducted in a Thermo VG Scientific Sigma Probe system using amonochromatic Al Kα (1486.6 eV) x-ray source in bulk and surface modes.Re4f, W4f and C Is core levels spectra were collected with pass energyof 20 eV. The spectrometer binding energies (BE) was calibrated bysetting the 4f_(7/2) core level of Au to 84.0 eV. Curve fitting was doneby the XPSPEAK 4.1 software using Voigt functions convolution with aShirley-type background subtraction.

FIG. 5a-5b show the results of C1s core level peaks, with their detailedde-convoluted bonding components for diamond:H covered with ReO₃ (3 a)and WO₃ (3 b) at increasing thicknesses and, as a reference, for a typeIIb boron doped diamond:H after annealing at 400° C. (lowest curve). Thedifferent C1s de-convoluted lines arise from a pure bulk diamondcomponent (blue line) at a binding energy (BE) of 284.2 eV, a surfacecomponent C—H (green line) chemically shifted to a higher binding energyby 0.25 eV, hydrocarbon contaminant surface component (C—H_(x) orangeline) shifted to higher binding energies by 0.58 eV, and a weak carboxyl(C—O pink line) feature in agreement with previous work (Russell et al.,2013). The presence of hydrocarbons (C—H_(x)), presumably originatingfrom respective oxide surface contaminants, becomes significant with theincreasing coverage. The shift of the diamond bulk peak position withincreasing ReO₃ thickness indicates a maximum shift of 0.9 eV from thebulk VBM position at about 34 Å. A maximum diamond bulk peak positionshift of 1.3 eV is found for the corresponding increasing WO₃ thickness.

Study 2. Diamond:H/WO₃ Metal-Oxide-Semiconductor Field-Effect Transistor(MOSFET)

The present Study demonstrates the first implementation of ametal-oxide-semiconductor field-effect transistor (MOSFET) utilizing thediamond:H/WO₃ system, and investigates the impact of WO₃ thickness, gatelength and low temperature operation on the device characteristics.

Device Fabrication

FIG. 6 shows a schematic cross section of the starting heterostructureand the fabricated MOSFET. Three 3×3×0.5 mm³ type IIa (001)-orientedsingle-crystal diamond substrates supplied from Element6 Ltd., withnitrogen concentration <1 ppm were used. Surface hydrogenation wasperformed by exposure to pure hydrogen plasma in a CVD reactor at 600°C. for 40 minutes. Subsequently, the samples were heated at 350° C. todesorb H₂O molecules and contaminants from the diamond surface in vacuum(Vardi et al., 2014). This was immediately followed by low-rate (0.1Å/min) thermal evaporation of different thicknesses (2.4, 3.4 and 4.8nm) of WO₃ in each sample. A surface roughness of Ra 0.6 nm over an areaof 1×1 μm was measured by AFM, and a WO₃ thickness uniformity of 10% wasevaluated by ellipsometry. The stoichiometry and thickness of WO₃ wascharacterized by X-ray photoelectron spectroscopy and ellipsometry andresults similar to those shown in Study 1 were obtained.

The process continues with electron-beam evaporation of Ti/Au (20/200nm), as source and drain electrodes, through a shadow mask. Followingthis, 20 nm of HfO₂ as gate dielectric layer was grown by atomic layerdeposition (ALD) at 150° C. The use of a gate oxide was deemed essentialto obtain a working FET since WO₃ is expected to become highlyconductive after the surface transfer process. After that, flowableoxide (FOX) was spin coated on the sample surface and exposed byelectron-beam lithography (EBL). This forms a hard mask that is used todefine the active channel. Reactive-ion etching based on a BCl₃/Cl/Archemistry was performed to etch the exposed HfO₂ and WO₃ and to desorbthe Hydrogen from the diamond surface. FOX was then striped with abuffered oxide etchant. Subsequently, a standard photolithographicliftoff step was used to create a Ti/Au (10/100 nm) gate electrode atthe center of the channel of the FETs. Devices with gate lengths (L_(g))ranging from 0.7 to 5 μm and a constant gate width (Wg) of 20 μm werefabricated. The source-drain distance gradually increased from 29 μm to58 μm as the gate length changed from 0.7 μm to 5 μm.

In this first demonstration, no effort was given to bringing the sourceand drain ohmic contacts directly onto the D:H surface. Rather, the goalof this work was to demonstrate the viability of the D:H/WO₃ as a MOSFETsystem.

Results

FIG. 7 shows electrical characteristics of typical MOSFETs with L_(g)=4μm and different WO₃ thickness. All devices show well saturated draincurrent behavior with sharp pinchoff and low output conductance (FIGS.7A and 7B). The 2.4 nm WO₃ MOSFETs show greater drain current, Id, andtransconductance, g_(m) (FIG. 7B), and a more positive thresholdvoltage, V_(T). These results are consistent with recent Hall effectobservations of decreased surface transfer efficiency and a reducedsheet hole concentration (from 2.5×10¹⁴ cm⁻² to 1.3×10¹⁴ cm⁻²) of D:Hwith increasing WO₃ thicknesses (from 1.2 nm to 4.8 nm).

The thinner WO₃ devices also show greater gate leakage current, I_(g),(FIG. 7B). This also results in worse drain current saturation (FIGS. 7Aand 7B). A more effective electron transfer into the 2.4 nm thick WO₃layer is a plausible explanation for the larger gate current.

We have studied the impact of gate length, L_(g), on the electricalcharacteristics of 4.8 nm-thick WO₃ devices. This is graphed in FIG. 8.FIG. 8A shows that the subthreshold behavior rapidly improves as L_(g)increases. This probably stems from a combination of short-channeleffects and reduced I_(g). In addition, we observe a significantimprovement in peak transconductance, g_(mmax), as L_(g) decreases (FIG.8B). The threshold voltage, V_(T), shifts positive as gate lengthshortens. This could be explained by severe short-channel effects thatarise from the use of a relatively thick gate dielectric coupled withthe absence of body doping (del Alamo, 2017). FIG. 8C plots g_(mmax) andthe maximum drain current, I_(Dsat) as a function of gate length atV_(DS)=−6 V. The results indicate both g_(mmax) and I_(Dsat) correlatewell with each other and scale in a well-behaved manner with L_(g).

In addition, we extracted the ON resistance of 4.8 nm-thick WO₃transistors with different L_(g) at VGS=−2 V and V_(DS)=−2 V (FIG. 8D).At this VGS, the ON resistance is largely saturated to its minimumvalue. From extrapolation of these data to L_(g)=0, we estimated a totalsource/drain access resistance of 197 kΩ·μm.

We have also studied the effect of temperature on the electricalcharacteristics of an L_(g)=5 μm, WO_(3=4.8) nm FET at 77 K. The resultsare presented in FIG. 9. At 77 K, we observe a large increase in I_(d)and g_(m) with g_(mmax) scaling up by 3.5 times. We also see that I_(g)was reduced by about two orders of magnitude (FIG. 9B). This results insignificantly improved subthreshold behavior with the minimumsubthreshold swing (S_(min)) scaling down from 1225 mV/dec to 190 mV/decand the ON-OFF ratio improving from ˜103 to ˜106, as the temperature isreduced from room temperature to 77K.

Device operation at 77 K was further studied by carrying outcapacitance-voltage (C_(g)—V_(GS)) and I-V transfer (I_(d)-V_(GS))characteristics in a device with 4.8 nm of WO₃ and L_(g)=5 μm. The C-Vcharacteristics were measured at 1 MHz with V_(DS)=0 V. I_(d)-V_(GS)measurements were performed at V_(DS)=−2 V. A typical C_(g)-V_(GS)result is shown in FIG. 10. From these data, we extract the gate voltagedependence of the sheet hole concentration (p_(s)) and hole mobility(μ_(p)) (Radisavljevic et al., 2011; Balendhran et al., 2013). We alsoassume that the access resistance does not change with temperature sinceit is believed to be largely dominated by the contact resistance. Wefind that, in any case, the extracted mobility exhibits weak sensitivityto the actual value of access resistance that was used.

The inset of FIG. 10 graphs the sheet hole mobility (μs) vs. sheet holeconcentration (p_(s)). Over most of its range, the mobility increaseswith hole concentration. This suggests that Coulomb scattering dominatesat low temperature. We observe a maximum sheet hole concentration ofabout 3.1×10¹² cm⁻², The corresponding mobility, however, is 1.8cm⁻²/V·sec. This is much lower than results obtained from Hallmeasurements at room temperature of similar unprocessed samples. This isalso consistent with Coulomb scattering that could be due to gap statesintroduced as a result of WO_(3-x) reduction during the devicefabrication process (Greiner et al., 2012). The reduction in resistancethat is observed as the temperature drops could be due to aninsulator-to-metal transport transition recently reported by Mattoni etal. in WO_(3-x) (Mattoni et al., 2017). This would also result in areduced work function and degraded surface transfer doping efficiency atroom temperature (Greiner et al., 2012).

The results of this Study reveal the potential of the D:H/WO₃ system forfuture transistor applications but also illustrate the challenge ofmaintaining high TMO quality during device fabrication, an issue alreadynoted in Vardi et al. (2014). To exploit the advantageous properties ofthe D:H/TMO system, transistor fabrication processes will need to bedeveloped that maintain the integrity of the TMO layer.

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What is claimed is:
 1. A conducting material comprising a carbon-basedmaterial selected from a diamond or insulating diamond-like carbon,having a hydrogen-terminated surface and a layer of rhenium trioxide(ReO₃) coating said hydrogen-terminated surface.
 2. The conductingmaterial of claim 1, wherein said diamond is polycrystalline diamond,nanocrystalline diamond, ultra-nanocrystalline diamond, or singlediamond.
 3. The conducting material of claim 1, wherein said rheniumtrioxide layer is a single- or multi-layer of rhenium trioxide having athickness in a range of 5 Å to 1000 Å.
 4. The conducting material ofclaim 1, wherein said carbon-based material is diamond and the thicknessof said rhenium trioxide layer is in a range of 5 Å to 1000 Å.
 5. Theconducting material of claim 4, having (i) an electrical stability of upto at least 450° C.; or (ii) a sheet conductance higher than 10⁻⁵Ω⁻¹,and a hole carrier concentration higher than 10¹² cm⁻².
 6. Theconducting material of claim 5, having a sheet conductance in a range of10⁻⁵Ω⁻¹ to 10⁻²Ω⁻¹, and a hole carrier concentration in a range of 10¹²cm⁻² to 10¹⁴ cm⁻².
 7. An electronic component comprising a conductingmaterial according to claim
 1. 8. An electrode comprising a conductingmaterial according to claim
 1. 9. The electrode of claim 8, comprisingsaid conducting material as a conductive layer.
 10. A sensor comprisinga conducting material according to claim
 1. 11. In a method fordetection of a chemical or biological material using a sensor, theimprovement wherein said sensor comprises a conducting materialaccording to claim
 1. 12. A diode comprising a conducting materialaccording to claim
 1. 13. The diode of claim 12, comprising a p-njunction, a p-n-p heterojunction or n-p-n heterojunction, wherein one ormore of the p-type layers comprises said conducting material and/or saidconducting material bridges said p-n junction, p-n-p heterojunction orn-p-n heterojunction; or configured as a schottky diode.
 14. A fieldeffect transistor (FET) comprising a conductive material according toclaim 1, as either a conductive layer or a p-type layer.
 15. The FET ofclaim 14, constructed as a high-frequency high power FET.
 16. A fieldemission electron source comprising a conductive material according toclaim 1.